Method of processing materials using an inductively coupled plasma

ABSTRACT

A method for coating surfaces or implanting ions in an object using an inductively coupled plasma. The method provides a gas-free environment, since the plasma is formed without using a gas. The coating material or implantation material is intitially in solid form.

BACKGROUND OF THE INVENTION

The invention disclosed herein is generally related to a method of usinga high frequency induction plasma tubes, and more specifically to amethod for processing materials using an inductively coupled plasma.This invention is the result of a contract with the Department of Energy(Contract No. W-7405-ENG-36).

High frequency induction plasma tubes are well-known for producing hightemperature gaseous plasmas. Such plasmas are useful in a number ofpractical applications including high temperature spectroscopic studies,preparation of microcrystalline refractory materials, and makingpowders.

In conventional methods of forming metal vapors using a plasma tube,metal, usually as a gaseous compound or powder, is introduced into agaseous plasma. The plasma is formed from an ionizable gas such asargon. The metal vapor is expanded through a nozzle and deposited on acold surface. In a similar manner, the metal vapor can be converted to avery fine powder.

This method of forming powders and coating surfaces has major drawbacks.A starting material cannot be introduced into conventional plasma tubeswithout onerous processing. Specifically, the starting material must beeither liquefied or formed into an aerosol or gaseous compound.

A second disadvantage of conventional methods of forming powders is thata gas is necessary in order to form the plasma. The gas promotes voidswhen the plasma tube is used for metal coating surfaces.

The reason that gas is necessary is that induction plasma tubes known inthe art generally include an electrical induction coil surrounding anenclosure which contains an ionizable gas. The coil is connected to asource of high frequency (400 kHz to 5 MHz) electrical power. A quartztube centered inside the coil typically defines the enclosure. It isbelieved that this type of conventional "induction" plasma tube isactually capacitively coupled with the electrical induction coil and thegas being one of the capacitor plates and the quartz tube being thedielectric material. If metal is used to form the plasma in this type ofplasma tube, the metal deposits on the quartz wall. The metal, ineffect, becomes part of the dielectric material and prevents the plasmafrom "seeing" the alternating electric field produced by the coil.Almost immediately, the metal deposit burns through the quartzenclosure.

Upon application of power to the induction coil in conventional plasmatubes, the gas is ionized producing a central core of hot gaseous plasmainside the enclosure. At low power levels the plasma is concentrated inthe center of the enclosure such that there is reduced danger of heatdamage to the enclosure walls. At high power levels, however, the plasmacore is both hotter and larger in diameter. As a result, the quartzenclosure is easily damaged by the plasma, which typically attainstemperatures on the order of 10,000° C. and above. This problem isaggravated by the fact that the plasma is typically subject to magneticand electrical instabilities that cause it to fluctuate in position andoccasionally actually contact the enclosure walls. Operation at highpower levels also results in the emission of intense ultravioletradiation from the plasma, which ionizes the air around the enclosureand may result in electrical arcing in the induction coil.

These adverse effects have been overcome by the use of internalwater-cooled shields as described and disclosed in U.S. Pat. No.4,431,901 issued on Feb. 14, 1984, the teachings of which are herebyincorporated by reference. By using a thick segmented shield shaped incross section to occlude line-of-sight transmission of light, it ispossible to get induction heating of the plasma because a current isinduced around each of the individual segments. Without occludingline-of-sight transmission of untraviolet radiation from the plasma, theair around the windings is ionized which induces arcing of the plasma. Acounterflow cooling system is used to cool the individual segments. Suchan improved shielding system makes it possible to maintain a plasma attemperatures on the order of 10,000° C.

U.S. Pat. No. 4,431,901 does not suggest the use of the plasma tube forcoating materials or for making ultrafine ultrapure powders. Inaddition, this patent does not contemplate the use of a pure metalplasma and, in fact, only mentions the use of an ionizable gas to formthe plasma.

Accordingly, it is an object of the present invention to provide amethod for producing ultrafine, ultrapure powders in a gas freeenvironment.

It is also an object of the present invention to provide a method ofcoating material with a pure metal.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the present invention includes the steps of forming a plasmausing a solid material and collecting resulting vapor on a coldcollection surface. In another aspect of the invention the enclosure ofthe induction plasma tube is evacuated. By evacuating the enclosure theuser can make a gas-free, contaminant-free metal powder. In anotheraspect of the invention, a pure metal or ceramic plasma is formed byusing a pure metal or ceramic ingot as the material introduced into theplasma tube. The resulting vapor is expanded onto a cold surface to formeither pure ultrafine powders or pure metal or ceramic coatings.

One advantage of the invention is that a solid material may be used as astarting material for making powders. This eliminates the need forspecial processing to convert the solid material to a powder or gaseouscompound.

A second advantage of the invention is that a gas-free plasma can beformed. A gas-free plasma produces higher quality coatings and powders.

These and other aspects of the invention are more fully set forth in thefollowing detailed description of the preferred embodiments and in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a side schematic of the induction plasma tube used with theinvention;

FIG. 2 is a plan view in cross section of the plasma tube illustrated inFIG. 1, taken along section line 2--2 of FIG. 1.

FIG. 3 is a schematic showing the device used for forming powders.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a cross-sectional schematic of the induction plasma tubeused to process materials which includes a water-cooled copper inductioncoil 10 which surrounds a tubular quartz enclosure 12. The enclosure 12extends upwardly from a water-cooled base 14 to an upper assembly 16which includes water supply manifold 18 and water exhaust manifold 20.Supply and exhaust manifolds 18 and 20 include annular interior waterchannels which are connected to exterior supply and exhaust waterfittings. Likewise, base 14 includes annular interior water coolingchannels which are connected to one another and which are connected toexterior water supply and exhaust fittings. Base 14 and manifolds 18 and20 are all annular so as to define central cylindrical cavity 22 whereina plasma may be formed by application of a high frequency electricalcurrent to induction coil 10.

The induction plasma tube used to practice the present inventionincludes segmented shield 30 which consists of twelve substantiallyidentical thick-walled copper tubes 32. The tubes are affixed at theirupper ends to water exhaust manifold 20 and extend downwardly therefromalong the inside of the tubular quartz enclosure. Water supply tubes 42extend the length of copper tubes 32 and are used to cool the individualshield segments. This can be seen best in FIG. 2. Details of the kind ofplasma tube used to practice the invention are described in U.S. Pat.No. 4,431,901.

The plasma tube is typically operated at a frequency of 400 kHz to 5MHz, at a power level of up to 50 kW applied to the induction coil.

Using a segmented shield induction plasma tube, a solid material can beused as the starting material for making powders and sprays. The solidmaterial may be a solid ingot, a mass of agglomerated material, looselypacked particulate or any other solid or quasi-solid form. The purity ofthe end product is directly related to the purity of the startingmaterial. Therefore, it is an advantage to use a pure solid, such as apure metal ingot, as a starting material. The need of processing toconvert the solid to a powder or gaseous compound is not necessary whenpracticing the invention. A solid material is simply placed on awater-cooled hearth. Because of the segmented shield device of U.S. Pat.No. 4,431,901 it is possible to introduce the solid material directlyinto the plasma.

In addition to the use of solid ingots as a starting material, purevapor (for example, pure metal), gas free plasmas can be made accordingto the present invention using the device disclosed in U.S. Pat. No.4,431,901. This can be done because the device is not capacitivelycoupled as are conventional induction plasma tubes. Therefore, theenclosure may be evacuated and the vapor of the solid material itselfcan form the plasma.

In addition to enabling pure vapor plasmas to be formed, it has beenfound that the induction plasma tube taught by U.S. Pat. No. 4,431,901can be electrically grounded. Conventional plasma tubes cannot begrounded because in such tubes the work coil is capacitively coupled tothe plasma and consequently there is a high radio-frequency voltage onthe plasma. If there is a ground near the tube, the plasma will beextinguished. If there is a more remote ground, the result may be severevoltage breakdown or arcing which would render the system inoperable.The device disclosed in U.S. Pat. No. 4,431,901 provides very littlecapacitive coupling because the shield effectively blocks the elecricfield of the coil. Consequently, the voltage on the plasma is so lowthat these problems just don't exist. In operation, the only element notgrounded is induction coil 10.

The plasma is formed from a starting ingot 70 shown in FIG. 3. Theplasma used to make pure powders is formed from a solid sample which isheated when a high frequency current is applied to an induction coil. Itis not necessary that there be any ionizable gas such as argon for theinvention to be used.

Using this method, the starting material, usually a pure solid, isvaporized by heating to form the plasma. This avoids the need for theargon or other ionizable gas used in conventional induction plasmatubes. Because a solid material is used as the starting material it iseasy to control the purity of the final product. In fact, the purity ofthe powder product could conceptually be equal to the purity of thesolid starting material.

FIG. 3 shows a material processing application which utilizes theinductively coupled plasma. The embodiment shown in FIG. 3 is for use inmaking ultrapure, ultrafine powders from solid ingot 70. A similarconfiguration may be used for coating applications. An ingot of solidmaterial 70 is placed on a water-cooled copper hearth 71. When inductioncoil 10 is supplied with power the ingot is vaporized, forming plasma72. The plasma, a metal or ceramic vapor, expands through nozzle 73 andsettles on cold surface 74. Because the solid sample material 70 is usedto form plasma 72, there are no impurities introduced into the system.Central cylindrical cavity 22 is evacuated in this embodiment to assurethat there are no impurities.

Although FIG. 3 shows nozzle 73 to expand the vapor, nozzle 73 is not arequirement to practice the invention. While hypersonic expansionprovided by a nozzle enables vapor to cool quickly, material tends todeposit around the mouth of nozzle 73. If nozzle 73 is used, it may benecessary to heat the nozzle area.

In a second application of a vapor plasma, metal or ceramic coatings canbe made by depositing the vapor onto surfaces. The thickness of thecoating is a direct function of time.

This aspect of the invention enables coatings with a controlledcomposition to be deposited onto a surface. Pure coatings can bedeposited without problems associated with gas plasmas.

As discussed above, it was discovered that segmented plasma tube 32(FIG. 1) used to practice the invention may be grounded. This enablescoating surface or target to be at one potential and the plasma to be ata different potential. By using different potentials. ion implantationsor high-volume deposition can be accomplished. For example, a negativevoltage can be applied to the target to attract the positive ions in thevapor. The deposition efficiency is thus increased because less vapormisses the target. Also, the bias may be used to accelerate the ions forion implantation.

The foregoing description of the methods of the invention have beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. For example, although the discussion above is limited toultrapure powders, it is also possible to make powders of controlledcomposition. Another possible application of the invention is to formgas-free plasmas from waste materials thus reducing their chemical bondsto relatively harmless atomic constituents. The methods and uses of theinvention were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claimed appended thereto.

We claim:
 1. A method of coating a surface in a gas-free environmentusing an electrically conductive ceramic or metal coating material whichis initially in solid form comprising:a. providing inductively coupledplasma apparatus comprising: (1) an electrical induction coil having acentral longitudinal axis; (2) a dielectric tubular enclosure centeredcoaxially on said axis and located inside said coil; (3) a segmentedmetal radiation shield which is grounded and centered coaxially on saidaxis inside said enclosure, said shield consisting of a plurality ofelongate fluid-cooled metal shield segments extending parallel to saidaxis, said segments being disposed in a circular arrangement adjacentthe interior surface of said enclosure and being substantially equallyspaced apart circumferentially such that said shield has a generallytubular configuration, and said shield segments being shaped in crosssection so as to occlude line-of-sight transmission of light throughsaid radiation shield; and (4) an outlet for said coating material; b.placing said coating material within said radiation shield; c.evacuating said tubular enclosure; d. heating said coating material byuse of said induction coil to vaporize said coating material and to forma plasma from the resulting vapor; and e. providing said surfaceadjacent to said outlet of said plasma apparatus, in order that materialemanating from the outlet is deposited on said surface.
 2. A method ofimplanting ions in an object in a gas-free environment using anelectrically conductive ceramic or metal implantation material which isinitially in solid form comprising:a. providing inductively coupledplasma apparatus comprising: (1) an electrical induction coil having acentral longitudinal axis; (2) a dielectric tubular enclosure centeredcoaxially on said axis and located inside said coil; (3) a segmentedmetal radiation shield which is grounded and centered coaxially on saidaxis inside said enclosure, said shield consisting of a plurality ofelongate fluid-cooled metal shield segments extending parallel to saidaxis, said segments being disposed in a circular arrangement adjacentthe interior surface of said enclosure and being substantially equallyspaced apart circumferentially such that said shield has a generallytubular configuration, and said shield segments being shaped in crosssection so as to occlude line-of-sight transmission of light throughsaid radiation shield; and (4) an outlet for said coating material; b.placing said coating material within said radiation shield; c.evacuating said tubular enclosure; d. heating said implantation materialby use of said induction coil to vaporize said implantation material andto form a plasma from the resulting vapor; and e. providing said objectadjacent to said outlet of said plasma apparatus, in order that ionsemanating from the outlet are implanted in said object.
 3. The method ofclaim 2 further including applying a negative voltage to said object,thus attracting positive ions.